1. Field of the Invention
The present invention relates to ultraviolet radiation detectors in general and more specifically to a solar blind ultraviolet radiation detector; that is, a detector insensitive to the radiation of sunlight reaching the earth""s surface but sensitive to UVC wavelengths in the spectrum, defined as the interval 200 nm-280 nm.
2. Description of the Prior Art
The sun emits in a broad spectrum of wavelengths including wavelengths in the ultraviolet as well as visible and infrared. However, the region of interest in this application is the ultraviolet band. Ultraviolet radiation emitted from the sun and reaching earth surface, as well as other non-natural UV sources, are a precursor of chemical reactions by breaking bonds between the constituent atoms of molecules. The radiative effects of ultraviolet radiation play an important role in biological molecules. Ultraviolet photons have the ability to excite the electronic states of molecules and probably induce photochemical reactions in molecules such as DNA, RNA or proteins in general. The ultraviolet light is divided into three regions, the UVA [320 nm-400 nm], the UVB [280 nm-320 nm], and the UVC [200 nm-280 nm]. However, the sunlight that reaches us on the ground does not contain any of the UVC portion of the light because these wavelengths are absorbed by the ozone layer in the stratosphere. UVC can only be created artificially, for example, by discharge in gases or by highly energetic flames. The UVC bandwidth [200 nm-280 nm] of the ultraviolet portion of the spectrum consists of highly energetic radiation that is capable of destroying bacteria, viruses and other microorganisms.
Many sources emitting in UVC are available. One example is a mercury lamp that has an intense emission line at a wavelength of 253.7 nm. Other UVC sources include excimer lasers (emitting at wavelengths such as 248 nm), xenon lamps, deuterium lamps, intense flames, arc welding, and energetic combustion such as in the plume of a missile or a rocket.
Applications of a solar blind detector or a UVC detector are as numerous as the sources of UVC. The solar blind terminology is derived from the fact that the ozone layer in the stratosphere completely absorbs the solar radiation of wavelengths shorter than 280 nm and the only naturally available wavelength ranges of ultraviolet radiation are the UVA and UVB. Therefore a detector limited to sensing only the UVC part of the spectrum is called a solar blind detector.
An advantage of using a solar blind detector versus using a broad band ultraviolet detector under sunlight is that the solar blind detector is not affected by the amount of natural fight available and remains unperturbed. Weak signals in the solar blind region can, therefore, be detected with high signal to background ratio. The presence of natural light does not affect the solar blind sensor.
Solar blind detectors based on semiconductors or photomultipliers are currently available in the market. Existing photomultiplier technology is bulky, fragile and relatively expensive, thus limiting their range of applications. General constructional and operational features of UV solar blind detectors of this type have been described in the relevant literature (see, for example, U.S. Pat. No. 4,731,881 and, U.S. Pat. No. 5,021,668). Semiconductor technology based on nitride optoelectronic materials offers a better alternative. However, they are still relatively expensive and they rely on an electrical power supply with an electronic circuit for signal amplification. For example, Nitres, Inc., of West Lake Village, Calif. is developing AlGaN photodiode technology for solar blind applications. Furthermore governmental agencies such as the Defense Advanced Research Projects Agency (DARPA) of the Department Of Defense (DOD) is presently developing solar blind detectors under the research program on nitride optoelectronic materials and devices.
The present invention pertains to a solar blind ultraviolet radiation detector; that is, a detector insensitive to the radiation of sunlight reaching the earth""s surface but sensitive to UVC wavelengths in the spectrum, defined as the interval 200 nm-280 nm.
In contrast with the prior art, the present invention is based on chemical sensors that do not require any electrical power supply or electronic amplification system. The sensing of ultraviolet is accomplished with the use of photochromic chemicals that react reversibly to ultraviolet by changing from a clear state to a colored state. The general behavior and applications of such chemicals as solar UV detectors have been described in my previous inventions: U.S. Pat. Nos. 5,581,090 and 5,914,197. Therefore, the application of photochromic compositions intended for the detection of the ultraviolet present in sunlight will not be described here.
As previously mentioned, UVC may be used to kill bacteria in environments such as water or air, for instance. Therefore, an obvious use for such solar blind UV detector is to monitor the output of any UVC source used for decontamination of water and air (water treatment and air purification) or for sterilization of biological materials (e.g., medical instruments).
An aspect of the present invention is to provide a method for making a Solar Blind-UVC detector including providing a photochromic compound having at least one of a spirooxazine and a spiropyran molecule and a chromene derivative, providing an optically clear ink selected for its propriety of minimal absorption in the UVC region, mixing the photochromic compound with the optically clear ink to produce a photochromic-ink composition, applying the photochromic ink in a form of a layer, on a substrate material, drying the layer with heat. The method further includes selecting UVA and UVB absorbing chemicals, mixing the said UVA and UVB absorbing chemicals with the optically clear ink to create a UVA and UVB blocking composition, applying the said blocking composition in a thin layer on top of the previously applied photochromic ink layer, and drying the UVA and UVB blocking layer to form a final UV sensing area where only UVC is detected.
In one embodiment the method includes producing a photochromic ink composition comprising dissolving a photochromic compound into an organic solvent and mixing the obtained solution with an optically clear ink matrix.
In another embodiment, the method includes producing a photochromic ink composition by adding ultraviolet light stabilizers and anti-oxidants to the said optically clear ink matrix.
Another aspect of the present invention is to provide a solar blind-UVC detector including:
(a) a photochromic chemical selected from the group of spiropyrans, spirooxazines and chromene derivatives, wherein said photochromic chemical is dissolved in an organic solvent and mixed to an optically clear ink, to form a photochromic composition which is applied to a substrate selected from plastic, paper, glass and metal. The optically clear ink is selected from a group of inks having the propriety of not absorbing in the UVC region of the spectrum;
(b) ultraviolet light selective chemical absorbers selected from the group of UVA and UVB absorbing chemicals. The UVA and UVB absorbing chemicals are dissolved in an organic solvent and mixed to said optically clear ink to form a UVA and UVB blocking composition which is applied on top of the previously applied photochromic composition.
It will become clear through a detailed description of preferred embodiments that the present invention teaches a UVC or solar blind detector with the manipulation of photochromic chemicals and chemical wavelength-selective blocks. It will also become clear that the UVC or solar blind detector is calibrated to indicate the amount of UVC radiation the UVC sensor area is exposed to.